No, the federal government has left their status the same - it was the State that created a special exception. If the IRS had obliged by allowing them to file the same as married couples do, it would have been tantamount to Federal recognition of homosexual "marriage" and would have opened the door for them to have their way in all states. If the States want to give them a break, that doesn't mean the rest of us should be forced to follow suit.
Polygamists can't file "Married, filing joint-joint-joint-joint-joint-joint" either...